The conventional charger used for two or more chargeable batters usually adopts a parallel charging method. According to this method, when only one chargeable battery is charged, the conventional charger can work normally, but when two or more than two chargeable batteries are parallel charged, the batteries must first be discharged to zero the electric capacities and thereby compensate for the different individual residual electric capacities of the batteries, after which the batteries are parallel combined for simultaneous charging. As a result, the following inconveniences are caused: 1) the discharge operation is time-consuming while the battery working life is reduced, and the electric power is wasted; and 2) if some of the chargeable batteries have been put in the charger and have been charged for some time, and another battery with a lower electric capacity is then inserted for parallel charging, the charging time for the originally-inserted batteries is prolonged accordingly.